/Engine/Source/Editor/UnrealEd/Classes/Editor/Transactor.h
Begins a new undo transaction. An undo transaction is defined as all actions which take place when the user selects "undo" a single time. If there is already an active transaction in progress, increments that transaction's action counter instead of beginning a new transaction.
Number of active actions whenBegin()was called; values greater than 0 indicate that there was already an existing undo transaction in progress.
virtual int32 Begin ( const TCHAR * SessionContext, const [FText](API\Runtime\Core\Internationalization\FText) & Description )